The present invention refers to a method and an apparatus for circulating solid material in a fluidized bed reactor, including a reactor chamber, having side walls defining the interior of the reactor chamber and a grid at the bottom of the reactor chamber; a gas discharge opening adjacent the upper end of the reactor chamber and a fluidized bed of solid particles in said reactor chamber, the fluidized bed having an internal circulation of solid particles.
In fluidized bed reactors, both conventional bubbling bed reactors and in circulating bed reactors, there is an internal circulation of solid bed material within the reactor chamber. The bed material is in a continuous upwards and downwards motion. The finer the solid particles the more easily they flow upwards in the reactor chamber. A fractionation of solids therefore takes place in the reactor chamber. A dense solid particle fraction, including larger objects, is formed in the lower part of the reactor chamber, whereas less dense solid particle fractions, including fine particles, are formed higher up in the reactor chamber.
It would sometimes be desirable to be able to process particles in the lower part of the reactor chamber, without having larger objects mixed therein. E.g. recovering heat from solid particles in the lower part of the reactor chamber would be easier if large objects could be prevented from reaching the heat transfer surfaces. Large objects tend to clog the heat transfer surfaces and also cause damage mechanically.
It is therefore an object of the present invention to provide a method and an apparatus for classifying solid particles within the internal circulation of solid material in a fluidized bed reactor.
It is further an object of the present invention to provide an improved method and apparatus for recovering heat from solid particles in the lower part of a reactor chamber, by minimizing pluggage of heat transfer surfaces.
In order to achieve the above objects, a method for circulating solid material in a fluidized bed reactor, having an internal circulation of solid particles, is provided in which the following steps are practiced: (a) Classifying solid particles in the internal circulation by arranging a particle chamber in the fluidized bed of solid particles, the particle chamber having at least one barrier wall having openings allowing only solid particles of a size smaller than a predetermined size to flow from the fluidized bed through the barrier wall into the particle chamber. (b) Guiding large objects, having a size bigger than the predetermined size of particles, downwardly in the reactor chamber externally of the particle chamber. And, (c) recirculating at least a portion of the particles from the particle chamber into the reactor chamber.
An apparatus for classifying solid particles in a fluidized bed reactor having an internal circulation of particles is also provided. According to the invention the reactor comprises the following elements: A particle chamber arranged in the fluidized bed of solid particles; the particle chamber having a barrier wall provided with openings, such as holes or slots, preventing solid particles of a size bigger than a predetermined size from flowing from the reactor chamber into the particle chamber; and, the particle chamber further having a wall provided with at least one opening for recirculating particles from the particle chamber into the reactor chamber.
According to a preferred embodiment of the invention heat transfer surfaces are arranged in the particle chamber. Heat is then easily and efficiently recovered from particles without large objects adversely affecting heat recovery.
The particle chamber may be arranged adjacent to a side wall or a partition wall in the lower part of the reactor chamber for classifying solid particles flowing downwardly by gravity along the walls. Then the uppermost end or roof construction of the particle chamber may form a barrier wall therein. The barrier wall may be horizontal or inclined. The barrier wall, having openings allowing only particles of a size smaller than a predetermined size to flow therethrough, prevents large objects from flowing into the particle chamber. By making the barrier wall inclined, e.g. about 30.degree.-45.degree. from horizontal the large objects are caused to continue to flow downwardly along the external side of the barrier wall without clogging the openings in the barrier wall.
A side wall of the particle chamber may in some embodiments form the barrier wall therein. Then particles having a horizontal momentum can flow through the openings in the barrier wall into the particle chamber.
The present invention may be used e.g. in fluidized bed combustors, where one or several particle chambers are arranged on the bottom of the combustion chamber. The particle chamber or chambers may be adjacent the side walls or partition walls in the combustion chamber or may even stand freely on the bottom. In some embodiments particle chambers may be arranged as protrusions higher up in the combustion chamber.
If the particle chambers are arranged adjacent one or several of the reactor chamber side walls, then a portion of the vertical or inclined side wall may form a common wall between the particle chamber and the reactor chamber. The particle chamber may be arranged on the inside or on the outside of the side wall part, which forms the common wall. Thereby the particle chamber may form a protrusion on the inside or outside of the reactor chamber. If the particle chamber forms a protrusion on the outside of the reactor chamber side wall, then the openings allowing particles to flow into the particle chamber are advantageously made in the portion of the side wall forming a common wall between the fine material chamber and the reactor chamber.
In hot surroundings the particle chamber may be constructed of water tube panels as is the reactor chamber itself. The tube panels may be refractory lined. The openings in the wall forming the barrier wall may then be made in fins combining adjacent water tubes or may be made by bending a water tube or two adjacent water tubes to provide a slot between the tubes. If the barrier wall is refractory lined then a furrow may be formed in the refractory lining and the openings may be made in the bottom plane of the furrow. There may be only one opening or slot in a barrier wall if the amount of particles flowing through this opening is sufficient. Usually several openings or slots are arranged in the barrier wall for securing a sufficient flow of particles.
Slots or rows of openings arranged in horizontal or inclined walls on top of the particle chamber may advantageously be arranged perpendicularly to the reactor side wall. In vertical barrier walls the slots or rows of holes are preferably arranged vertically, but may in some embodiments be arranged horizontally.
The particle chamber may e.g. reach to a level 3 to 8 meters above the grid of a circulating fluidized bed reactor, so that a rather large downwards flow of particles may be caught by the particle chamber.
There may be long particle chambers covering substantially the whole length of a side wall or several side walls, or there may only be one or two small particle chambers arranged spaced apart from each other at a side wall.
The openings in the barrier wall may in a circulating fluidized bed combustor be holes, having a diameter smaller than about 50 mm, preferably about 30 mm or smaller, or a slot having a width smaller than about 50 mm, preferably approximately 30 mm or smaller. Such openings allow only approx. round particles of a size &lt;50 mm, preferably smaller than 30 mm, or oblong particles having a width &lt;50 mm, or preferably smaller than 30 mm, to flow through the barrier wall.
In fluidized bed combustors the particle chamber may be used for heat recovery. Then evaporators, superheaters or other heat transfer surfaces are arranged in the particle chamber. The invention provides a possibility to run combustors at low load, even when it is impossible to get enough heating capacity in the upper regions of the combustor chamber or in external heat exchangers. The invention makes it possible to obtain a balance between superheating and evaporation for different loads or for different fuels.
Heat transfer surfaces may be arranged in any conventional manner in the particle chamber. The heat transfer may be controlled by introducing fluidizing air/gas into the fine material chamber. The fluidizing air may be used as secondary air in the combustion chamber.
A good mixing of solid material in the particle chamber is important if heat is to be recovered from the particles therein. This can be emphasized, if desired, by arranging the inlet or inlets and outlet or outlets of solid particles in opposite ends of the particle chamber.
According to a preferred embodiment of the invention particles are recirculated from the particle chamber back into the reactor chamber. The particles may be recirculated by overflow through one or several openings in the particle chamber. The openings may be arranged on one side wall of the particle chamber or there may be openings on several side walls. It is in most cases advantageous to arrange the openings far away from the inlet openings in the barrier wall, if a good mixing of particles in the particle chamber is desired.
The particles may alternatively be recirculated through a gas lock, such as narrow slot like openings disposed one above the other in a side wall in the particle chamber. The particles may also be recirculated through a gas lock of an L-valve type formed between the particle chamber and the reactor chamber. The recirculation may be controlled by fluidizing fine particles in the vicinity of the gas lock. The particles may of course also be recirculated into the reactor chamber by mechanical means, such as a screw feeder.
Fluidizing air introduced into the combustor, for control of heat transfer or for transporting particles into the particle chamber, may be used as secondary air in the combustion chamber. The particle outlet openings or the particle inlet openings allow gas to flow outward through the barrier wall countercurrently to the particle flow. The particle flow inward into the particle chamber is unstable and does not prevent gases from escaping out from the chamber.
In a circulating fluidized bed reactor bed material is discharged with the exhaust gases and separated from the gases in a particle separator. The particles are thereafter reintroduced through a particle inlet into the reactor chamber, usually into the lower part of the reactor chamber. When utilizing a particle chamber according to the present invention in a circulating fluidized bed reactor, the externally circulated bed material may be wholly or partly reintroduced into the reactor through that particle chamber. An inlet for externally circulated material is then provided in the particle chamber. If heat transfer surfaces are to be arranged in the particle chamber, then heat is recovered from both externally and internally circulating material.
Further features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of the drawings, and from the appended claims.